1. Field of the Invention
The present invention relates to a method for the production of a glass product. More specifically, it relates to a method for the effective production of a glass product having a surface polished highly accurately, particularly a phosphate glass product or a fluorophosphate glass product, as well as a filter formed of the glass product obtained by the above method, and a solid image sensor device for which the filter is adapted.
2. Explanation of Related Art
Generally, glass-containing phosphate as a glass network former has excellent transmission in an ultraviolet region, and it is therefore used in fields where the transmission in an ultraviolet region is important. A colored glass (color filter glass) containing phosphate as a glass network former has been more stably colored with a transition metal ion such as Fe.sup.2+ or Cu.sup.2+ than a color filter glass containing SiO.sub.2 or B.sub.2 O.sub.3 as a glass network former, and is therefore used in various purposes.
Meanwhile, for improving glass in stability, optical constants, transmission characteristics and chemical durability, it is general practice to add alkali metal oxides such as Li.sub.2 O, Na.sub.2 O, K.sub.2 O and Cs.sub.2 O, alkaline earth metal oxides such as CaO, MgO, SrO and BaO, other divalent metal oxides such as ZnO and PbO or F to a glass. When the above components are added as required, the glass can have stability sufficient for shapability and can be mass-produced without causing devitrification.
It is known that the above glass is used for an infrared absorption filter which is a spectral luminous efficiency correction filter of CCD (charge coupled device) for use, e.g., in a color VTR camera. The glass used for the above filter is imparted with the property of absorbing light having a longer wavelength than 700 nm by incorporating CuO as a colorant thereinto and utilizing the absorption by Cu.sup.2+ ion. In this case, the Cu.sup.2+ exhibits excellent absorption only when phosphate is used as a main component of a glass network former. For the above filter, therefore, there is used a phosphate glass or a fluorophosphate glass to which CuO is incorporated. The glass is polished so as to have a thickness of approximately 1.0 to 2.0 mm, and is used as a filter for an image sensor element such as CCD. In the image sensor element, the demand for high density has been increasing, and an area per pixel of photodiode is exceedingly decreased. There is therefore a phenomenon that even a flaw or scratch having a size of the order of several micrometers which has not caused any problem so far causes a detrimental effect on an image. For example, a flaw having a size of greater than about 7 .mu.m takes or occupies the region for almost one pixel of photodiode, and an image defect in a solid image sensor device is observed. When an infrared absorption filter is disposed close to CCD and when the number of pixels per unit area is large, a filter for use in the above field is therefore required to have a highly accurate polished surface (polish flaw width .ltoreq.7 .mu.m).
The above phosphate glass containing phosphate as a glass network former essentially has a poor glass structure, and it is therefore liable to have polish-induced flaws and is easily chemically reactive. However, an increase in the hardness of the glass is limited in terms of the glass composition, and unlike a borosilicate glass, it is difficult to obtain a hardness sufficient for easy polishing. When desired transmission characteristics, chemical durability, glass stability adequate for mass-producibility and other optical characteristics are intended to be maintained, an improvement in the composition is limited. It is therefore difficult to impart a phosphate glass or a fluorophosphate glass with a hardness which a borosilicate glass has, and most glasses of this type is so-called least polishable glass having a low hardness. There are therefore limits placed on stably obtaining a highly accurate polished surface (polish flaw width .ltoreq.7 .mu.m) for the glasses of the above type even if the glass composition is improved or the polishing technique is improved.
For polishing the above phosphate glass or fluorophosphate glass, conventionally, there is employed a method in which the glass is polished with a polishing liquid prepared by adding an abrasive such as CeO.sub.2 to water. Generally, as the load for polishing is decreased or as the rotation rate for polishing is decreased, the accuracy of the polished surface of a glass having a low hardness increases. However the phosphate glass and the fluorophosphate glass not only have a considerably low hardness, but also are highly chemically reactive, and therefore, they have the following defects. They show limits in polish accuracy, latent flaws are liable to occur, and it takes a long period of time to polish them.